NBS Measurement Services:

Calibration of

Beta-Particle-Emitting

Ophthalmic Applicators

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National Bureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support

to the National Measurement Support System-for ionizing radiation; conducts research in radiation related

fields to develop improved radiation measurement methodology; and generates, compiles, and critically

evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the • Atomic Spectroscopy

spectroscopic and radiative properties of atoms and highly ionized • Atomic Radiation Data

species; develops well-defined atomic radiation sources as radiometric • Plasma Radi at ion or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division ^ ^ Provides the central national basis for the measurement of far ultra- • Far UV Physics

violet, soft x-ray, and electron radiation; develops and disseminates • Electron Physics

radiation standards, measurement services, and data for for these radi a- • Photon Physics

tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applications; determines fundamental mechanisms of electron and photon interactions with matter; and develops advanced electron- and photon-based

measurement techiques. The Division consists of the following Groups:

Radiometric Physics Division

Provides national measurement standards and support services for ultra- • Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination • sjaectrophotometry

and measurement quality assurance services; conducts research in optical • Radiometric Measurement Services

radiation, pyrometry, photometry, and quanttm radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

• Accelerator Research

• Linac Operations

• Electronic Instrunentation

• Mechanical Instrinentation

in the standardization of nuclear instrunentation; and develops

new competence in radiation sources and instrunentation. The

Division consists of the following Groups:

Develops, operates, ana improves major Nbi raaiation raci inies

including the electron Linac and race track microtron; develops,

designs, and builds electronic and mechanical instrunentation for

raHiatinn nrnnrarK: 3nH fflrilitip«;r nrnvidp"; natinnal Ifadprshio

Ionizing Radiation Division

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Provides primary national standards, measurement services, and basic

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and measurement support services to the National Measurement Support

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

understanding of physical processes that underlie measurement science.
NBS MEASUREMENT SERVICES:

CALIBRATION OF BETA-PARTICLEEMITTING OPHTHALMIC APPLICATORS

John S. Pruitt

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1987
Library of Congress Catalog Card Number: 87-619844

National Bureau of Standards Special Publication 250-9

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-9, 36 pages (July 1987)

CODEN: XNBSAV

Commercial products— materials and instruments— are identified in this document for the sole pur- pose of adequately describing experimental or test procedures. In no event does such identification imply recommendation or endorsement by the National Bureau of Standards of a particular product; nor does it imply that a named material or instrument is necessarily the best available for the purpose

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (6) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document (SP 250-9), NBS Measurement Services: Calibration of

Beta-Particle-Emitting Ophthalmic Applicators, by J. S. Pruitt, is the

ninth to be published in this new series of special publications. It describes the calibration of ophthalmic applicators used in medical

dosimetry for treatment of conditions that can be corrected with the

shallow-dose exposures available from beta-particle radioactive

sources. The calibrated quantity is the surface absorbed-dose rate to

water averaged over the active area of the source (see test number

47030C in the SP 250 Users Guide). Inquiries concerning the technical

content of this document or the specifications for these services

should be directed to the author or one of the technical contacts cited

in SP 250.

The Center for Radiation Research (CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

George A. Uriano Chris E. Kuyatt

Director Director

Measurement Services Center for Radiation Research

1 ^ 1
ABSTRACT

This service provides calibrations for ^^Sr + beta particle

ophthalmic applicators. The calibration determines the average

surface absorbed-dose rate to water over the active area of the

applicator. The technique used is to measure current per unit mass

of air at the active surface with an extrapolation ionization

chamber, and to convert this into absorbed-dose rate with Bragg-Gray

cavity ionization theory. The extrapolation chamber measurements

are made in three parts. In the first part, a 1 mm diameter probe

electrode is used to map the relative dose rate across the source surface, and to determine the active area. In the second part, the

probe electrode is replaced by a 30 mm diameter electrode, and the

test applicator is replaced by an NBS-owned applicator for quality

assurance measurements. In the third part, the 30 mm electrode is

used with the test applicator to determine the extrapolation curve

slope at the source surface. The ratio of this slope to the source

area, divided by the air density, yields current per unit air mass.

The transformation to absorbed-dose rate is described. The

component uncertainties are listed and are shown to yield a total

uncertainty of ± 15%, with the approximate significance of a 95%

confidence limit. Data book measurements and a calibration report

are given for one particular applicator.

Key words: absorbed dose rate; calibration service; extrapolation chamber;

ophthalmic applicator; quality assurance measurements; surface

dose rate.

iv
CONTENTS

Page

Abstract iv

List of Figures v1

1. The Calibration Service 1

2. The General Approach 2

3. Description of the System 3

4. Operational Procedures , . . . 10

4.1 Current measurement and correction 10

4.2 Measurement of source area A 12

4.3 Quality assurance measurements 16

4.4 Current per unit air gap 18

5. Surface Absorbed-Dose Rates 19

6. Calibration Uncertainty 20

References 21

Appendix A. Examples from a databook A-1

Appendix B. Sample calibration report B-1

V
LIST OF FIGURES

Page

Fig. 1. NBS-owned ophthalmic applicator SN 0258 1

Fig. 2. Electron energy spectrum from a ^^Sr + ^^Y source 2

Fig. 3. Cross section of 30-mm diameter collecting electrode unit,

showing relative position of high voltage electrode and

applicator 3

Fig. 4. Front view of extrapolation chamber and source support structure 5

Fig. 5. Side view of extrapolation chamber and source support structure 6

Fig. 6. Cross section of 1-mm diameter collecting electrode unit ... 7

Fig. 7. Front view of extrapolation chamber housing and measuring

instruments 9

Fig. 8. Variation of ion current and parasitic current across applicator SN 0136 11

Fig. 9. Currents measured with the probe electrode across the face of

SN D-402, plotted as a function of radius, measured from the

center of the current distribution 14

Fig. 10. Theoretical probe profiles for five different source distances,

n

= Az/R 15

Fig. 11. Measured probe profiles of applicators SN 0258 and SN 157, in

comparison with theoretical predictions for uniformly

distributed sources 16

Fig. 12. Front view of extrapolation chamber showing source support

structure retracted for quality assurance measurements .... 17

vi
1. The Calibration Service

This service is intended to provide calibrations for ^^Sr + betaparticle applicators used in medicine for treatment of ophthalmic conditions.

It was started in 1977. A typical ophthalmic applicator is shown in figure 1. It is a New England Nuclear Model NB-1. The National Bureau of Standards

(NBS) owns an NB-1, serial number 0258. The quantity chosen for calibration

is the average surface absorbed-dose rate to water over the active area of the source. If an applicator is immersed in water, or placed on a water-like

material , such as tissue, the surface absorbed dose rate to water is the rate

at which energy is absorbed by water from the beta particles, per unit mass of

water, at the surface of the applicator. The applicators accepted for

calibration generally have activities between 370 MBq (10 mCi ) and 7.4 GBq

(200 mCi ) . (The corresponding range of surface absorbed-dose rates is from

about 0.1 Gy/s to about 2 Gy/s.) Activities less than 370 MBq are not very

useful because of thejong times required to obtain biologically significant

exposures, while activities larger than 7.4 GBq would- produce radiation

protection problems for the persons using and calibrating the applicator. The

stated uncertainty is ± 15%, with the approximate significance of a 95%

confidence limit.

Welds

A 0.08 mm thick SS foil

B Porous glass disc infused with

radioactive material

C Aluminum Spacer

D 6.35 mm diameter handle

E 10cmX10cmX5 mm plastic shield

Fig. 1. NBS-owned ophthalmic applicator SN 0258.

1
This service was called 8.4K in the old SP250 manual. The designation

has been changed to 47030C in the 1986 version of SP250.1 The latter also

contains instructions for shipping and handling the applicators.

A report entitled "Calibration of Beta-Particl e Ophthalmic Applicators at

the National Bureau of Standards" has been published in the Journal of

Research of the NBS.2 It contains much of the information given in this

report. Examples from a data book are given in Appendix A, and a sample

calibration report is given in Appendix B.

2. The General Approach

The average surface absorbed-dose rate to water is determined from

ionization-chamber measurements in air, and converted to water with Bragg-Gray

cavity-ioni zation theory, a routine conversion described in detail in

section 5. Water was the medium chosen for calibration because it is

universally available and its properties are close to those of tissue. The

primary alternative method of calibration uses thermoluminescent dosimeters,

but this method was rejected because these dosimeters must be calibrated with

a different radiation source. Use of the Bragg-Gray theory requires knowledge

of only a well-known constant, the energy required to produce unit ionization

in air, and the ratio of average stopping powers for water and air. This

ratio can be calculated from tabulated stopping powers if the beta-particle

energy spectrum is known. Figure 2 shows a typical strontium 90-yttrium 90

energy spectrum. It was obtained with a 5-mm-deep silicon surface-barrier

detector at room temperature in air, under 0.013 mm of aluminized polyethylene

terephthal ate. The measurements were made at a distance of 11 cm from a Buchl er-Amersham source, and have not been corrected for either backscatter or self-absorption. The spectral shape need not be known with great precision,

however, since stopping-power ratios vary slowly with beta-particle energy.

1600 1

1 1

1 1 1 r

Electron Energy, MeV

Fig. 2. Electron energy spectrum from a ^osp + goy source.

2
The determination of average surface absorbed-dose rate is performed in

two steps using ionization chambers of different diameters, because two

quantities need to be measured. The total current per unit air gap at the source surface, (I/d)^, is measured with an ion chamber at least two

6-particle ranges larger in diameter than the active area of the source; the

active area A of the source is determined by mapping with an ion chamber much

smaller in diameter than the diameter of the active area. In practice, the

two measurements are made with the same ionization chamber, but the size of

the collecting electrode is changed. The average surface absorbed-dose rate

is directly proportional to the ratio (I/d)p/(pA), where p is air density.

This ratio has dimensions of current per unit mass of air (A/kg).

The general plan of the calibration is to measure ionization current per

unit mass of air with the applicator at several distances from the center of

the ionization chamber air gap, and to extrapolate to zero distance and zero air gap to obtain a value at the surface of the applicator. Sections 3 and 4 are concerned with this determination of extrapolated current per unit mass of

air. Section 5 describes the calculation of the absorbed-dose rate, and

section 6 is a discussion of the uncertainties inherent in this calibration

method.

3. Description of the System

The extrapolation ionization

chamber is essentially the same

as that described by Loevinger

and Trott,3 with several

collecting electrodes which can

be interchanged and a thin

al umi ni zed-pl asti c-f i 1 m highvoltage electrode. The relative

position of the electrodes and an ophthalmic applicator under study

are shown in figure 3. The

extrapolation chamber itself is a parallel plate capacitor of

variable plate separation, d.

The guard electrode is grounded

and the collecting electrode

potential is very close to

ground. In this condition, the

lines of force are perpendicular

to the collecting electrode, and

the collecting electrode receives

only charged particles that have

been generated in the cylindrical

air volume defined by d and the

collecting electrode area, A^.. The extrapolation chamber has means for both changing d and for

measuring this change, but does

not provide an absolute measure

of d itself without extrapolation

Fig. 3: Cross section of 30-mm diameter

collecting electrode unit,

showing relative position of

high voltage electrode and

appl i cator.

3
The particular collecting electrode shown has a diameter of about 30 mm

and is surrounded by a guard electrode with an outside diameter of 50 mm. Both guard and collecting electrodes are made of 8-mm thick A-150 conducting

plastic, electrically isolated with 30-um plastic film held in place with

epoxy cement. The high-voltage electrode is Mylar aluminized on the inside

surface, with a diameter of 70 mm and a thickness of 0.7 mg/cm^ (about 5 i\m)

,

Figures 4 and 5 are respectively a 3/4 front view and a side view of the

apparatus, showing the extrapolation chamber above and the NBS ophthalmic

applicator SN0258 below. (The applicators in figures 4 and 5 have been

retracted from their normal position by about 3 cm for clarity.) The extrapolation chamber housing is mounted on the underside of the 6-mm thick

triangular brass top plate, which is supported on three 19-mm diameter brass

posts. The lower half of this housing, including the bronze ring on which the

circumferential scale is engraved, the captain's wheel, and the mylar HV

electrode foil, can be rotated to change d, the chamber plate separation.

Each complete revolution of the captain's wheel changes d by 0.635 mm

(40 turns/inch). The internal collecting-electrode-guard-electrode unit of

figure 3 cannot be seen in figure 4, but it rests on three brass supports

rigidly attached to the top plate in figure 4.

The orientation of the captain's wheel, uniquely related to the plate

separation d, can be specified by a single number, the angle of rotation

(which may increase to many multiples of Zir radians). In practice, that angle

is broken down into two related components, 9 and g. The number e is one- hundredth of the number read from the circumferential scale shown in figure 4.

This scale consists of 100 divisions, so e varies from 0 to 1 in one revolution. The number g is read from the dial gauge visible at the top of

figure 4. There are two scales on the dial gauge. The large scale is divided

into 100 divisions, with 0.001 inches per division. The small scale is

divided into 10 units, with 0.1 inches between units. The dial gauge reading

g is the sum of the small gauge reading, a whole number, and the large gauge

reading, a fraction between zero and unity. Its units are tenths of an inch.

The two numbers g and e can be related by setting 9 to zero and then

rotating the dial gauge until the large needle points to a multiple of 0.25

(four revolutions of the captain's wheel are equivalent to one revolution of

the large dial gauge needle). Then the electrode separation, in mm, can be

written

d = 2.54 (gg - g) where 9 = 9i - f

(D

and gg is the dial gauge needle reading when d = 0. g^ is the closest

multiple of 0.25 larger than g (for two examples, if g = 5.173 (or 4.635),

g^ = 5.25 (or 4.75), and e = 0.308 (or 0.460)). gg must be measured by

extrapolation, as described in the next section.

The collecting electrode shown in figure 3 has a much larger area than

the active area of all ordinary ophthalmic applicators, which have diameters

between 6 mm and 9 mm. This type of collecting electrode is used to determine

the ratio (I/d)Q, where I is the ionization current generated in the air

volume defined by the collecting area A^. and the electrode separation d. The

ratio I/d is a function of both d and the applicator distance Az, and the zero subscript refers to the limiting case where both d and az are zero. This

limiting value of I/d is a characteristic of the source and of the electrode

material and is independent of A^., if A^- is sufficiently large.

4
5
• Extrapolation chamber

Fig. 5: Side view of extrapolation chamber (above) and source support

structure. NBS applicator SN 0258 is shown.

6
For measurements of the source

area A, the collecting-electrodeguard-el ectrode unit of figure 3 can

be lifted out of the extrapolation

chamber and replaced by a unit with

approximately the same outline but

with a much smaller collecting electrode, shown in cross section in the

upper diagram in figure 6. This

probe electrode is a 0.6-mm-di ameter

pencil lead, shown as a solid black

vertical strip in both upper and

lower parts of figure 6. It is

surrounded by a 0.4-mm-thick insulating ring of epoxy resin, giving an effective probe diameter of about

1 mm. The probe is surrounded by a graphite guard ring, 19 mm in

diameter. This in turn is sur- rounded by a 50-mm-di ameter acrylic

ring that is recessed from the

surface of the graphite by about

0.4 mm. The graphite guard ring is

grounded through a graphite coating

on the acrylic ring.

The probe electrode is more

complex than the 30-mm electrode

because of the special precautions

taken to make the 0.4-mm epoxy ring

of uniform thickness. The lower

diagram in figure 6 shows how this

was achieved with alignment holes

which were eventually removed. The

alignment holes are in graphite,

outside the two Finish Planes,

completely filled by the pencil

lead. After the epoxy has dried,

the unit is machined until it looks

like the graphite-epoxy insert in

the upper diagram. It was necessary

insulator, from which the bubbles had

centri fuge.

Fig. 6: Cross section of 1-mm diameter

collecting electrode unit (top)

and the technique used to

center pencil -lead collecting

electrode (bottom). After

machining, only the material

between the two finish planes

remains.

0 use a slow-drying epoxy for the

been removed by spinning in a

The ophthalmic applicator can be moved in three perpendicular directions.

The x and y motions (along axes shown in fig. 3) are provided by the microscope stage, and are controlled by two horizontal rods with knurled knobs at

the ends. These rods are a safety measure, allowing the x and y positions to

be set to within ± 0.05 mm without excessive radiation exposure to the

fingers. Motion along the z axis (perpendicular to the x-y plane) is con- trolled by the vertical barrel micrometer shown just to the right of the

applicator in figure 4. While changing az, the operator's fingers are protected both by the brass plate on which the microscope stage is mounted and

by the 1/4-inch-thick plastic shield attached to the applicator handle.

7
During measurements of both (I/d)^ and A, the source surface must be

raised until it touches the extrapolation chamber high-voltage-electrode foil.

(This does not produce a short circuit since only the inner foil surface is at

high voltage.) For determination of the zero-separation dial -gauge reading

gg, or for source insertion and removal, the source support structure has to

be retracted by about 20 cm. These large changes in the z coordinate are produced by releasing the three rapid retraction screws shown in figure 5 and

sliding the microscope-stage down on the three vertical steel rods shown. The

barrel micrometer is used only for fine control of az.

Figure 7 shows both the apparatus and the measuring instruments. The

apparatus, at the right in figure 7, is surrounded on three sides by a 3/8-

inch-thick transparent plastic housing, intended for radiation protection of

the operator who sits facing the front panel. His hands can be inserted into

the housing for source and/or chamber manipulation through doors on either

side. The operator's torso and face are always protected by 3/8-inch of

plastic. The applicator shown inside the housing in figure 7 has been covered

with a lead cap, to protect the operators fingers during source/chamber manipulation. This lead cap was put in position with the tongs shown lying on the

bottom plate of the apparatus in figure 7.

The extrapolation-chamber collecting electrode is connected via low-noise

coaxial cable to the input of a Keithley 610C electrometer. This instrument

has a dial readout that is usually turned off. In its place, a Digitec 266 DC

voltmeter (DVM) provides a digital readout of the electrometer feedback

voltage. The XI Output of the 610C is connected to the Hi terminal of the

DVM, and the Guard terminal of the 610C is connected to the Lo terminal of the

DVM. The 610C is used with the Feedback switch set to Fast, and the mode

switch set to lO'^, lO-^, or lO-io coulombs full scale.

Underneath the electrometer in figure 7 is a locally made DVM timer,

which controls the Digitec DVM display. The timer output is connected to

terminals D6 on the DVM card-edge connector, so that the DVM displays the

instantaneous electrometer feedback voltage only when it gets a positive pulse

from the timer. The front panel of the timer contains an ERC counter, an ITC

comparator, and a Start button. When the Start button is pushed, the timer

generates a pulse, the DVM displays an initial voltage, and the counter starts

counting elapsed time. When the counter reaches the pre-set comparator

number, it generates a second pulse, the DVM displays a final voltage, and the

counter stops counting. For the particular timer shown in figure 7, the time

unit for both counter and comparator is centi seconds. (For some of the local

DVM timers, the time unit is milliseconds.)

Underneath the Digitec DVM is a high-voltage power supply, connected

between the chamber high-voltage electrode and ground. This is a Keithley

Model 240A, which can provide from 0 to 1200 volts potential of either

polarity. (Although this power supply could be replaced by one delivering 0

to 100 V, both the amplitude and the stability of the 240A have proven

adequate.) The voltage is connected to the chamber high voltage electrode by means of the brass rod shown at the upper right in figure 5, which makes a sliding contact with a brass ring mounted on insulators above the captain's

wheel. The voltage selected in any given situation depends on the electrode

separation d. During extrapolation measurements, a constant potential

gradient is maintained, since this both minimizes the corrections and exerts a

8
E t_ O O)

i- (-> 4— O)

O) o

00 t_

•r- 4-> 3: O ^ OJ o .— o <u

on •>

-t-> c: ^

O) =» e o

Z3

— • £_ C_

-!-> t- (1) 00 OJ 4_> c -(-> o;

•r- a; e E o

C7> +-> L_

•rO

t_ >

=3 -a

V) I TfC fO o

<D -M t- E -r- OJ CD C

fT3 ^ .—- C\J

4_)

—

-c -a

•r— t_ (tJ —

c_ O) 4^ "

OJ

C7) E r— C O Q- E Qoo t- :3 3 0) on O -£r

jz: +-> £_

t-— "S OJ '—I o —' Q- E

fO O) O) ^ £_ cn o 13 fO

+-> cr 00 ,

O -M O

•1— c > E -C ,— 13 O) O -r- Q. 4-> ^ fO 00 L C ^ 4_) -I— tn X --^

4- +-> e. O OJ

•> E S +-> -r- OJ <4- +J

•r- (U > I—

C

C O) o o.^

u_

—

cn 9
constant force on the flexible high voltage electrode. It has been found that

electrode flexure is negligible if the voltage gradient does not exceed

40 V/mm. Measurements have been made successfully with gradients between

10 V/mm and 40 V/mm. Corrections have to be applied in all cases, as explained in a later section.

The two instruments at the extreme left-hand side of figure 7 are for

determining the relative air density inside the chamber. The lower instrument

is a Wallace and Tiernan Model FA139 aneroid barometer, which reads ambient

air pressure in units of millimeters of mercury (mmHg). The upper instrument

is a Yellow Springs Instrument Co. Tele-Thermometer, to which is attached a long cable with a temperature probe at the end. This probe usually sits in

the chamber well, immediately above the brass backing of the collectingelectrode-guard-electrode unit, and gives an approximate measure of the

chamber air temperature.

4. Operational Procedures

This section contains descriptions of the measurement procedures for both

current per unit air gap, (I/d)Q, and source area. A, as well as correction

formulas and quality assurance measurements. An example of data collection

and analysis is given in Appendix A, taken directly from databook 863.

Appendix B shows the report issued for this particular ophthalmic applicator.

4 . 1 Current measurement and correction

The extrapolation chamber current measured in amperes during a run is:

V - V

1 = 0.-4^ (2)

where: C is the electrometer capacitance, in farads,

Vj and Vp are initial and final DVM voltage measurements, in volts.

At is the run time, in seconds, determined by the DVM timer.

The value of the capacitance C must be determined for each electrometer

scale by independent measurements. For the particular electrometer shown in

figure 3, NBS #184539, the capacitances measured in March 1982 were:

Full-scale (C) Capacitance (pF)

10-8 10,000.

10-9 1,011.9

10-10 103.94

(These numbers have changed by several tenths of 1% from earlier measurements

in 1979.) Each voltage determination is the product of a DVM reading and a DVM correction factor. For the DVM of figure 7, NBS #183047, the correction

factor measured on 86 Feb 21 for the 2-V (or 20-V) scale is 0.9977 (0.9991).

(These have changed by 0.1% (0,1%) over the course of 3 years).

10
The measured current is the sum of two components, an ion current

generated in the chamber air gap by interaction between air molecules and the

incident beta-particles, and a parasitic current generated in the body of the

collecting electrode by beta-particles coming to rest. These two components

can be separated by making measurements with both positive and negative

high-voltage polarity. The ion current component will change sign when the

polarity changes, while the parasitic component will not.

If 1+ and I- are currents measured with positive and negative polarity

respectively, the ion current is:

where I~ is normally a negative number. The parasitic current is ( I"^ + I~)/2

and may be either positive or negative. It has no immediate application, but

is usually recorded because abrupt changes may indicate measurement problems.

The data tabulated in Appendix A are good examples of the normal range of

variation of the parasitic current, labeled Tg. Page 136"lists data taken

with a 30-mm diameter collecting electrode such as shown in figure 3. The Tg

measurements in this case show a normal progression from about -40 pA to about

-38 pA as the chamber air gap increases from 0.5 mm to 2.5 mm. Departures

from this progression by 20% or more usually indicate that one of the system

electronic components is malfunctioning.

Figure 8 shows the

variation of both the ion

and parasitic currents

across the face of the most

asymmetric applicator that

has been studied in this

laboratory. The measurements were made with the

probe electrode, using an electrode spacing, d, of

0.05 mm and an applicator

distance, az, of 0.4 mm.

The asymmetry of the ion

current is presumably

caused by an uneven dis^tribution of activity across

the applicator face. The

tails of the ion current

distribution are caused by

beta particles that leave

the applicator with large

velocity components perpendicular to the beam direction. The tails rapidly

become more prominent as the applicator distance Az

increases.

Fig. 8:

10 20 30

DISTANCE ACROSS SOURCE , mm

Variation of ion current and parasitic

current across applicator SN 0136.

11
The graph of parasitic current in figure 8 shows evidence both of

electrons coming to rest in the collecting electrode (the central peak of the

distribution), and background (the negative tails). The central peak of the

parasitic current is more symmetrical than the ion current peak, probably

because of multiple scattering of the primary beam in the plastic backing of

the electrode, which tends to round off the edges of the distribution. The

parasitic background is a current of about 10-50 fA. The ion current

background is negligible.

The ion current calculated with eq (3) must be multiplied by two

correction factors, one to correct air density inside the chamber to 22°C and

760 mmHg, and the other to correct for ion recombination and diffusion inside

the chamber. These factors are given by:

f ^ 760 (T + 273.15) , .^ ^temp 295.15 P

^

'

Where T is the temperature in degrees Celsius and P is the pressure in mmHg;

f rn 50.5 Id w., 0.0044 sr, in-suL-i-^Zlil^^i i'^ t a\

^ecom

= -""^^^^ -"Y~H1 - (17.24 X 10 5)( )]} (5)

where I is ion current in nA, d is electrode separation in mm, A is source

area in mm^, and X is potential gradient, in V/mm. Equation (4) comes from

the ideal gas law, and eq (5) comes from Bcrhm,** with bracketed terms for

general recombination, initial recombination, and diffusion, reading from left

to right. (For the data listed on p. 136 of Appendix A, as the chamber air

gap increases from 0.5 mm to 2.5 mm, the general recombination term decreases

from 0.9996 to 0.9911, the initial recombination term holds steady at 0.9998,

and the diffusion term increases from 0.9949 to 0.9990.)

4.2 Measurement of source area A

This operation is performed with the small collecting electrode, or probe

(fig. 6), using a fixed small air gap d (fig. 3), and with az just slightly

larger than d/2. Current measurements can be made at different points in the

x-y plane over a large enough area so that the current drops to less than half

its maximum value in all directions. If the x-y plane, determined by the

microscope stage, is parallel to the plane of the chamber high-voltage

electrode, this mapping can be done with no change in Az. These planes are seldom parallel, however, because the rapid retraction mechanism described in

section 3 (for large changes in az) is only a crude adjustment. With the

vertical position az set so that the applicator is close to but not touching

the HV electrode foil when the applicator is centered, the changes in x and y

that occur during mapping of the source may cause the applicator to run into

the electrode, diminishing the air gap in the chamber and reducing the chamber

sensitivity.

The operator can avoid this effect by simply sighting by eye to determine

when the applicator is close to the HV electrode foil, independent of the

barrel micrometer readings. A simple test can be run once, to get a feel for

what the position of the two components looks like when they are too close.

12
With the applicator and HV-electrode foil just barely separated by eye, the

operator can measure the ion-chamber current, reduce the separation by 0.01 mm

using the barrel micrometer reading, repeat the current measurement, etc. It will be found that the current increases slowly as Az is reduced, until the

two components touch. With further reduction of az, the current drops

rapidly.

For most ophthalmic applicators, the uppermost surface is not flat, as shown in figure 3, but slightly convex. The orientation of the applicator

should be such that the convex surface would be first to touch the HV electrode when AZ is reduced to d/2. In extreme cases, the head of the applicator

is cocked relative to the handle, so that the rim of the surface meets the HV

electrode first. In such cases, it may be possible to compensate for this

awkward alignment by tilting the source-support plate, depressing one or more

of the three fittings attaching it to the vertical steel rods.

In preparing for source-area measurements, it is necessary to find g^,

the d = 0 point on the dial -gauge scale. This can be done with the source in

position, but retracted by about 20 cm. In this condition, the corrected

current is a linear function of d, and the d = 0 point can be determined by

measurements with several air gaps and linear extrapolation to zero current.

The corrections required are the two listed in the preceding section, ^^q^q

and fpeQom* P^^^ ^ third factor that corrects for scatter from the sidewalls

of the chamber, and a fourth factor that corrects for attenuation in the air

between source and chamber. These extra factors are appreciable only when the

source-chamber distance is large. At 20 cm, the empirical formulas for these

factors are:

f ^ = 1 - 0.0094 d (6) scat ^

'

where d is the chamber air gap in mm, and:

f = (1 + 0.077 h - 0.335 h2)-i

atten ^

h = 1 - (f^ )-i . (7)

^ temp^

Equation (6) was derived by increasing the thickness of scattering material,

and eq (7) comes from experiments at reduced atmospheric pressure. ^ (There is no need to determine g^ each time the probe electrode is used. Its value may

change slightly when this electrode is removed and replaced, but the source

area measurement is insensitive to this change.)

The mapping itself consists of repeated measurements with different x and

y coordinates but with similar az = d/2. The air gap d should be reduced to

0.1 mm or 0.2 mm, with the chamber voltage gradient set for about 20 V/mm,

For each change of x and/or y, the applicator must first be retracted

(az > d/2) and then returned to az = d/2 after the change, because the x-y

plane and the HV foil plane are not parallel. The theoretical description of

the chamber current given in eq (8), below, can be used to show that small

variations in az have a negligible effect on the measured currents.

13
If Xq and y are the values of x and y with the applicator centered by

0 eye relative to the collecting electrode, it has been found that 36 current

measurements at '2-mm intervals over the ranges Xq-5 < x < x q"*"^ yQ-5 < y < Yq+S cover a large enough area so that the current drops to less

than half its maximum value in all directions. (In the Appendix A example,

the array of current measurements is plotted at the top of p. 135.) Along

each X = constant (or y = constant) line there are two values of y (or x) for

which I = Ij^^^/2. These two y^, (or X;,) values can be calculated by linear

interpolation, and their average is a measure of one coordinate of the center

of the distribution, y^. (or x^). This may or may not coincide with the

originally estimated center coordinate yQ (or Xq). At the end of the mapping

measurements, an average y^^

example, yi, and Xi, are listed

and X. are listed as y^,, and

c av The averages = 35.45 mm

and x^ can be calculated. (In the Appendix A on 135 as y~ and x~, while the values of y.

av c and x^ = 121.88 mm, are not far

from the original y^ = 36.0 mm

and Xq = 121.2 mm.) I is

plotted as a function of

r = /(x - X(-)2 + (y - yj-)2, as shown in figure 9 for a particular applicator. (In

the Appendix A example, this

plot is at the bottom of

p. 135.)

For most applicators, the

current decreases monotonical ly

with increasing radius as in

figure 9. A 50% radius R" can

be read directly from the

graph, and the area calculated

from this as TrR"^. This is a close approximation to the real area itR2 if the chamber spacing

d, and source distance Az, are both sufficiently small. A

rough estimate of uncertainty

for a given d and az can be

obtained from the following

equation describing the Fig. 9; ionization current per unit mass of air as a function of

position:

3.0

RADIUS, mm

Currents measured with the probe

electrode across the face of

SN U-402, plotted as a function

of radius, measured from the

center of the current distribution. The curve was drawn

freehand.

14
dm (8)

where in terms of the coordinates of figure 1, y = r/R, n

= Az/R,

r = /x2 + y2, and K is a constant.

This equation was derived by the author assuming that the activity is

spread uniformly over a circle of radius R on the source surface, that the

beta-particles are emitted i sotropi cal ly from the source surface, that their

paths are straight, and that energy deposition is uniform along these paths.

The derivation is based on an A/r2 solid angle approximation where A is a plane area, and consequently current per unit solid angle becomes infinite as the distance approaches zero. If the current/volume is normalized to its

value on the central axis of the source (y = 0), the normalized current is

well behaved for all distances (that is, for all values of n).

Figure 10 is a plot of the norma_lized currents for several values of n. For each curve, the apparent radius R is the product of the true radius R and

the value of jj where the curve crosses the half-value line. For n

= 1,

Az_ = R, and ttR2 exceeds t7R2 by a factor of about 2. For n

= 0.2 (not shown)

itR2 exceeds ttR2 by about 4%, and for n = 0.1, the difference is less than 1%.

Iri_ a typical case, d = 0.2 mm, Az = 0.12 mm, R = 4 mm, and n = 0.03, for which

TrR2/TrR2 = 1.001.

1.0

z

LlJ a:O

Q 0.5

UJN

a:o

z

1 1 1 1 1 1 —\ r—

1 1 1

yN/^s^-^r0.001

0 0.5 ! 1.5 2

RELATIVE RADIUS /X= r/R

Fig. 10: Theoretical probe profiles for five different source distances,

n

= Az/R.

15
SN 0258

THEORY

Figure 11 is a plot of probe ion current vs^. radius for two applicators and

matching source profiles calculated from eq (8). The parameter az was assigned the approximate value 0.1 mm in the calculation, and values of

R = 3.18 mm and 4.35 mm were read from the graph. All four curves have been

normalized at the origin. Equation (8) is not an accurate description of

either source profile, but the comparison with the NBS owned SN 0258 is consistent with the assumption

that the activity in this source is spread uniformly

over an area of 4.35-mm

radius. The small differences between theory and

measurement appear to be

easily explained. The

differences near r = R can be

attributed to the finite size

of the probe electrode. The

differences at larger radii

probably arise from the fact

that the activity in SN 0258

lies below the applicator

face, so that the elemental

sources cannot be treated as isotropic at large angles.

This will cause the real

distribution to fall off more

rapidly at large radii than

the theoretical distribution.

The activity in SN 157, in

contrast, appears to be

highly peaked at the center.

Fig. 11:

RADIUS

Measured probe profiles of

applicators SN 0258 and SN 157, in

comparison with theoretical predictions for uniformly distributed

sources.

4.3 Quality assurance measurements:

After measuring the source area, the probe electrode unit (fig. 6) should

be replaced by the 30-mm electrode unit (fig. 3), and the source support

structure should be retracted for a determination of gg, the d = 0 dial -gauge

reading, for this electrode. The unknown source shoula be replaced by the

NBS-owned SN 0258 applicator so that this determination can also serve as a quality assurance test. It is necessary to mount the applicator in a position

that is independent of the vertical position of the source support structure,

which is not reproducible. This can be done as shown in figure 12. The

SN 0258 plastic shield should be pushed as close to the source end as possible. The support structure should be retracted far enough so that the

applicator handle rests on the bottom plate in figure 10 when it is centered

in the hole as shown, but not far enough for the support structure itself to

touch bottom. In this condition, small vertical motions of the support

structure have no effect on the applicator position.

Extrapolation chamber current measurements should be made at each of the

five g settings listed below. The current measurements should be multiplied

by five correction factors, the four already described by eqs (4) to (7), as

well as a decay correction factor:

16
Fig. 12: Front view of extrapolation chamber (above), showing source support

structure retracted for quality assurance measurements. NBS

applicator SN 0258 is shown with its plastic shield moved towards the

source.

17
Way = exp (X At) (9)

where At is the time since the reference date of 1986 Jan 1, and x is the

decay constant (6.659 x 10" ^ dayi for 90sr).

Finally, Qq should be determined by fitting the corrected currents to the

equation:

I = Cq + c^g (10)

and calculating:

(11)

The numbers computed for I, Cq, c^, and g^ should be recorded. The data

listed below represents the first three sets of measurements made with the

configuration shown in figure 12.

Captain's wheel Current, pA

setting, g Feb 20 Mar 21 Apr 3

4.682 1.990 2.013 2.022

4.485 3.939 3.969 3.974

4.288 5.948 5.968 5.982

4.092 7.951 7.977 7.961

3.895 9.923 9.939 9.967

^0 49.288 49.271 49.316

^1 -10.106 -10.097 -10.105

So 4.877 4.880 4.880

The measured currents increased with time, but not enough to change the

coefficients Cq and c, or the di al -gauge-zero setting by more than 0.1%.

It is felt that all of these measurements were valid, but the reason for the

current increase is unknown. These periodic checks will continue, and the

results considered satisfactory unless g^ changes by as much as 1%. This

would correspond to a change of about 0.1 mm in the dial -gauge-zero setting.

4.4 Current per unit air gap :

Applicator SN 0258 should now be replaced by the unknown source, and the source support structure raised until the source is near the extrapolation

chamber high-voltage electrode. Then with d = 0.5 mm, and x = x^, y = y^., the

barrel micrometer reading for which az = d/2 = 0.25 mm should be found, by

decreasing az until the current drops rapidly. Current measurements are then

made for the 5 different air gaps d = 0.5, 1, 1.5, 2, 2.5 mm, with z = d/2 + 0.01 mm, for which the barrel micrometer readings are m = mg + d - 0.49. The

current measurements should be corrected by the factors f^g^p and ^^qqq^ from

eqs (4) and (5).

^

18
The corrected current per unit air gap at zero air gap, (I/d)g, comes from fitting the five corrected currents to the equation:

I/d = (I/d)o (1 + a d + 6 d2) (12)

This extrapolation is only approximate for this poor-geometry determination.

Sometimes a quadratic equation of this type fits the data very well and

sometimes poorly. In general, a » -0.1 mm-i, and 6 0.01 mm-2.

5. Surface Absorbed-Dose Rates

The average surface absorbed-dose rate in water comes from the Bragg-Gray

f ormul a

D ^ = ^^^TT^tH^ (I/d)n ^ ^ K , Gy/s (13) water 1.197 A 'o extrap back foil ^

'

where A is the source area in mm2 and (I/d)^ is the corrected current per unit

air gap at zero air gap, in nA/mm. In the above equation, 33.7 is the average

energy in joules, needed to produce one coulomb of ions of either sign in

air,^ 1,124 is the ratio of the average collision mass stopping power of water

to that of air, '7 and 1.197 is the density of air at the reference temperature

and pressure, in kg/m^. The correction factors are:

'^extrap

~ 0«98, a correction for systematic errors in the extrapolation

technique;

'^back " 1*010, a correction for difference between backscatter from water

and backscatter from TE plastic;

'^foil " 1*003, a correction for foil attenuation in the extrapolation

chamber entrance foil.

The (I/d)^ ratio from eq (12) depends on the extrapolation path to a minor extent. For example, if the ratio Az/d had been maintained at 1 rather

than at 1/2, the measured (I/d)Q ratio would have been slightly smaller.

Ideally, the ratio should be measured varying only d, with az = 0 throughout.

The closest one can come to this ideal situation is to extrapolate I/d to zero

d for several fixed values of az and then extrapolate the zero-d limits to zero AZ independently, the procedure described at the end of section 2.

However, this increases the calibration time by a large factor and has

consequently been replaced by the present techniques, where Az and d are extrapolated to zero simultaneously. For applicator SN 0258, it was found

that the ideal (I/d)^ is 2% smaller than the value obtained with Az/d = 1/2.

This is the origin of the correction factor kg^^-^^p.

The backscatter factor was estimated from published formulas for backscatter probability as a function of atomic number and beta-particle energy.

Using the chemical formulation for the collecting electrode material, and the source spectrum measured with the silicon detector, figure 2, the value for

'^back obtained.

The foil attenuation correction factor, k^Q^--] = 1.003, was determined

from measurements with extra thicknesses of 0.7 mg/cm2 aluminized foil.

19
6. Calibration Uncertainty

There is some uncertainty associated with each term of eq (13). Those

determined by extrapolation-chamber measurements with the 30-mm-diameter

collecting electrode, such as (I/d)Q and k^Q^--| , have (relative) standard

deviations of the order of 0.1%. These are much smaller than the estimated

uncertainties and are usually ignored. The estimated uncertainties are listed

in the table below. Each is intended to be comparable to a (relative)

standard deviation, in that the chance that the true value of the quantity

lies within plus or minus the stated uncertainty is intended to be about 2 out

of 3 (or, strictly speaking, there is a 2 out of 3 chance that the confidence

interval generated in this fashion will cover the true unknown value of the

quantity)

Rel ative

Absorbed -dose-rate uncertainties Uncertainty

(%)

Instrumental 0.3

Average energy per ion pair 0.4

Stopping-power ratio 3

Rate of change of current, (I/d)g 3

Extrapolation correction, k . ^ extrap

Backscatter correction, k^^^|^

Attenuation correction, k.

foil

Source surface area, A

1

1 <0.1

6

Combined uncertainty (quadratic sum) 7.5

Overall uncertainty (combined x 2) 15

The instrumental uncertainties include voltmeter readings, capacitance

measurements, and pressure and temperature determinations. The uncertainty in

W/e comes directly from reference 6. The stopping-power-ratio uncertainty

includes uncertainties in the stopping-powers themselves plus uncertainties in

the beta-particle spectra. The uncertainty in {1/6) q is an estimate of the

reliability of a quadratic fit to an extrapolation curve with Az = d/2. The

uncertainties in the three k factors are all estimates based on the size of

the corrections and on how carefully they were measured. The uncertainty in

surface area is a (relative) standard deviation, based on repeated measurements for several sources, using the normal 2-mm grid.

When the uncertainties are combined in quadrature to obtain a combined

uncertainty, the result is ± 7.5%. This number is an approximate (relative)

standard deviation. Doubling it gives an overall uncertainty of ± 15% which

can be interpreted as having the approximate significance of a 95% confidence

1 imi t

The surface area uncertainty is dominant in table 1 and is the only one

that can be appreciably reduced at present. It can be reduced to about 1% by

mapping a normal source with a 1-mm grid, but this step is so much more timeconsuming that it is not offered as an NBS calibration service. In this case,

however, the combined uncertainty would be about ± 4%. This would lead to a stated overall uncertainty of ± 8% with the approximate significance of a 95%

confidence 1 imit.

20
REFERENCES

1. Calibrations and Related Services, National Bureau of Standards,

Spec. Publ. 250, Gaithersbury , MD 20899 (1986).

2. J. S. Pruitt, "Calibration of Beta-Particle Ophthalmic Applicators at

NBS," J. Res. Nat. Bur. Stand. (U.S.), 91, 165 (1986).

3. R. Loevinger and N. G. Trott, Int. J. Appl . Rad. Isotopes 17, 103 (1966).

4. J. Bahm, Physics in Med. and Biol. 21, 754 (1976).

5. J. S. Pruitt, Radiation Protection Dosimetry Ij^, 151 (1985).

6. ICRU Report 31, International Commission on Radiation Units and

Measurements (Washington, D.C., 1979). The value of W/e cited comes from

the value for dry air (33.85 J/C), modified by a humidity factor for

ambient air taken from Figure 5.14.

7. M, Berger and S. Seltzer, National Bureau of Standards Internal Report,

NBSIR 82-2550A (1983); also as ICRU Report 37 (1984). The electron

spectrum used was measured with a lithium-drifted silicon detector.

8. Tabata, Ito, and Okabe, Nucl . Instr. Methods 94, 509 (1971).

21

APPENDIX A. Examples from a data book

The examples given are four pages from data book DB863, pages 133-136.

These cover the calibrations of applicator SN176. The first three pages cover

the area determination, using probe electrode 0.5A, and the fourth page covers

the determination of (I/d)g, using 30-mm-el ectrode 30F. There were no determinations of gg, the a = 0 dial -gauge reading, (The quantities called g and g^ in this report have in the past been called i and £q.)

The values of gg used in the calibration of SN176 had been measured on 1983 Nov 11 for electrode 0.5A (g^ = 5.585), and on 1985 Apr 1 for electrode

30F (gg = 4.888).

The databook column headings and their meanings are:

time,n :Real time on a 24-hour scale (i.e., 1436 is 2:36 p.m.). n is a double

number, a-b, where a is the number of initial runs not included in the

average and b is the number averaged (i.e., 1-5 means there were six

measurements, but only the latter five were averaged).

equip : A listing of the apparatus. On page 133, for instance, the numbers

stand for:

0.5 A is the collecting electrode number

SN 176 is the applicator number

185325 is the NBS number of a Keithley 610C electrometer

103.13 pF is the measured capacitance on the scale used (10~^° coul

)

168929 is the NBS number of a Fluke 412B high voltage power supply

189695 is the NBS number of the comparator on the DVM timer

25 s is the comparator setting

183047 is the NBS number of a Digitec 266 DVM

2V is the DVM scale used

calk:

A listing of the quantities necessary to calculate currents from

measured voltage changes. They are stored in the memory of an HP41C

calculator in bins:

81: d, the chamber electrode separation, in mm

82, 83 and 84: a, B, and k , constants needed for f and f

85: A, either the active area of the source or the collecting

electrode area, whichever is smaller, in min^

86: X, the extrapolation chamber potential gradient, in V/mm

87: I/I(nA), a constant defining the units of the quantity I 88: At, the number of days away from the reference date

89: A, the source decay constant, in days-^

90: C/t, the ratio of capacitance to runtime. C = C x 10^2^

t = t X lO**". The exponent k is usually chosen so that C/t is

a number between unity and 10.

91: Cg, the DVM calibration correction factor

A-1
z,d : Source distance and electrode separation, respectively, both in mm.

A, 9 (or g,e) : The extrapolation chamber coordinates needed to obtain the electrode

separation d. i (or g) is measured in tenths of an inch and 6 in

fractions of a revolution.

m,HV : m is the reading of the barrel micrometer used for fine adjustments of

the vertical position of the applicator, in mm. HV is the extrapolation

chamber potential difference in volts.

x,y : The applicator horizontal and vertical coordinates on the microscope

stage, in mm.

The correction factors f. and f

temp decay

If V is a voltage read from the DVM, V = V x 10"^, where j is chosen to

eliminate decimal points in recording the data. 6V is the average

change in V during a run. o-^ is the standard deviation per run of this

average.

Normalized ion current and parasitic current, respectively. From the

definitions ofj and k, 1=1 x lo

^'^^^'^

^ and is usually recorded without

decimal points.

The correction factors f . and f

scat recom

A-2
NBS S06 —Analyalf pap«r

90 1- 10?

GPO 9«7 0F— •404e«

U/»A^ / n

- /I /)

1 I A

-ir/jo

g>-. <^.a SN 17^^ . ( • 37« 7

bTi 6 77^ — -T7<?4 — ^ i77^

-;3 7

HI' o.HW €,Oo •f 32/ 177 iT — I7i

- 377 — - - Ho

- 7^/ 7f ._r3„ -

-^•^-^

7<

7/

2_'^5_ 17

1 \ - 7^

' -7

-

<?!"

- ^7

BMC ^ 33 ^7

11

- <7l ^

7^

'

~i/7

^l.i->jr- i 73

- . I -111

if ^3

-mi

A-3
13^

NBS 506 —Analytli paper

90 1- 10?

cro i»i2 of ^

1 J) *J

1 / \ / vn Hv ^ . -^T^h . ^

/^H^ 0,1 l|«,2. 3V ' -

-1-77

- - - 1

5", 75" 4. 7

-—

-

1 A 3 5

HI ^,L0 -t- <J27

-

11 - 2)1,0 Oil

5',C'5 + 11^ _

s:c7 4- /I

- —. - 7.066-

£'.11 f "l/t' —-•

- HZ

T S'M + v5 ,0

s.ci +

'2 C" A 11 Zb^

( 0 —

• —

I V » f

.

^/.^) - 5l 117

1

-ii

-i'l? " +

^11

' / ST;!, CrOS'Z

s,Ld~ \.

" " -is"?:'! L'hCL —

10

0 12fi

223

A-4
NBS 306 —Analysis pap«r

GPO t»«2 or — •«048«

lij WW]

A-5
NHS .'>06 —Analyilt paper

90 1- 10 • CPO 1 9«2 OF — *404*« r /

n r S- 4. ^ H

l=^== L ^ )l

A

1

^ / ^ p J / J

+ i\7

&n

...

.._

ici^l. l pF o ->

,

... .

•f —

- JAO

-14; ^

^—

_

• J^c^ „ ACA

.—

—.

^fj 1 _

»-

-- —

LC^

7.7c ^Z^'

- .

1

t -fjo r

^.1^7 +20

i<Hy Z «.7f_f \^\o \ Z- i y, /

1 \.66 7S

f IC^j

.-

i03C^ I -

"I —

\

) , CI ^

; 1 ,2.6 X _ j__ -' '"^^—rT"

.5/ \.0

JC (^^ (1,10 -iiC I. on?

-3^1

1-6177

uu

^

J 0

^

• , _

— '17 7

—/./f,

A-6
APPENDIX B

OG 8420/86

OB 863/133

TFN ?35324

1985 JUN 24

Pdge I of 2

IJ. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG. MO 20899

REPORT OF CALIBRATION

^^Sr Beta-particle Source

Submitted by

Technical Operations Source No,

RS #85-0879

176

Received at NBS on 1985 JUN 28

Calibration date 1985 JUL 10

Beta-particle sources are calibrated in terms of the absorbed-dose rate to

water. Absorbed-dose rate is determined from current measurements with an extrapolation ionization chamber that has a tissue-equivalent-plastic collecting electrode. The water absorbed-dose rate is given by

f) _ 33 .7 ^ 1 .1 24 X B X U f4li r^,/< ^ - 1.197 X A Ud^ ^^/^

where {AI/Ad)Q is the rate of change of current (normalized to a reference

temperature and pressure) with extrapolation chamber air-gap thickness as the

thickness approaches zero, 33.7 is the mean energy expended per unit charge in air,

1.124 is the ratio of the mean mass stopping power of water to that of air, 1.197

is the density of air at the reference temperature and pressure (22*C and 1 standard atmosphere), B is a correction for reduced backscatter from the collecting

electrode, U is a correction for attenuation by the high-voltage electrode, and A

is an area. For calibrations at the surface of the source, A is the active area of

the source, defined as t he a rea within the 50% isodose line at the source surface.

The listed diameter is 2/A/n

B-1
OG 84^0/85

1985 JUN 24

Pdtjp ? of ^

For the source submitted, one calibration was made, to determine the average

absorbed-dose rate to water across the surface of the source. The active

source area was determined by mapping the relative dose rate across the source

surface with a 1-mm diameter collecting electrode. The rate of change of current with air-gap thickness was measured with a 28-tnm diameter conducting

plastic collecting electrode.

The water absorbed-dose rate at the surface of the source, averaged over the

active area of the source, is given in the table in SI units (grays per second)

and in special units (rads per second).

This measurement has been assigned an uncertainty of ± 15%, which has the

approximate significance of a 95% confidence limit.

Information on the technical aspects of this report can be obtained from

J. S. Pruitt, Radiation Physics C206, National Bureau of Standards,

Gaithersburg, Maryland 208799. (301) 975-5587.

Calibration performed by J. S. Pruitt

Report approved by R. Loevinger

For the Director

by

William R. Ott

Chief, Radiation Physics Division

Center for Radiation Research

National Measurement Laboratory

Area Correction factors Water

Source Distance A Diameter B U Absorbed-dose rate

(mm) (mm^) (mm) (Gy/s) (rad/s)

176 0 46 7.6 1.010 1.003 0.43 43
FORM NBS-1 14A (REV. 1 1 84) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/SP-250/9

2. Performing Organ. Report No 3. Publ ication Date

July 1987

4. TITLE AND SUBTITLE InBS Measurement Services:

Calibration of Beta-Particle-Emitting Ophthalmic T^plicators

5. AUTHOR(S)

John S. Pruitt

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, Stote. ZIP)

Same as item 6.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number 87-619844

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT

This service provides calibrations for ^^Sr + ^^Y beta particle

ophthalmic applicators. The calibration determines the average

surface absorbed-dose rate to water over the active area of the

applicator. The technique used is to measure current per unit mass

of air at the active surface with an extrapolation ionization

chamber, and to convert this into absorbed-dose rate with Bragg-Gray

cavity ionization theory. The extrapolation chamber measurements

are made in three parts. In the first part, a 1 mm diameter probe

electrode is used to map the relative dose rate across the source surface, and to determine the active arpa . Tn tj^e seconc! pa^t the

proDe electrode is replaced by a 30 mm diameter electrode, and 'the

test applicator is replaced by an NBS-owned applicator for quality assurance measurements. In the third part, the 30 mm electrode is

used with the test applicator to determine the extrapolation curve slope at the source surface. The ratio of this slope to the source

area, divided by the air density, yields current per unit air mass.

The transformation to absorbed-dose rate is described. The

component uncertainties are listed and are shown to yield a total

uncertainty of ± 15%, with the approximate significance of a 95%

confidence limit. Data booK measurements and a calibration report

are given for one particular applicator.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

absorbed dose rate; calibration service; extrapolation chamber; ophthalmic

applicator; quality assurance measurements; surface dose rate.

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

[X^ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, DC

20402.

Order From National Technical Information Service (NTIS), Springfield, VA 22161

14. NO. OF

PRINTED PAGES

36

15. Price

{rU.S. Government Printing Office : 1987 - 181-100/6 25 24 USCOMM DC 85-4006



PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

SP 250-1 NBS Measurement Services:

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

SP 250-2 NBS Measurement Services:

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

SP 250-3 NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet

J.Z. Klose, J.M. Bridges, and W.r'. Ott

SP 250-4 NBS Measurement Services:

Fricke Dosimetry in HighEnergy Electron Beams

C.G. Soares, E.L. Bright and M. Ehrlich

SP 250-5 NBS Measurement Services:

Alpha-Particle Calibrations

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J, Hsia, K.D. Mielenz,

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg

SP 250-8 NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt

SP 250-10 NBS Measurement Services:

Radioactivity Calibrations with the "4tt" Ganma Ionization Chamber, and Other

Radioactivity Calibration Capabilities

J.M. Calhoun

SP 250-11 NBS Measurement Services:

Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L. McLaughlin

SP 250-12 .NBS Measurement Services: Neutron Personnel Dosimetry

R.B. Schwartz

SP 250-13 NBS Measurement Services:

Activation Foil Irradiation

with CaliforniLTi Fission Sources

G.P. Lamaze and J. A. Grundl

SP 250-14 NBS Measurement Services:

Activation Foil Irradiation by Reactor Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

SP 250-15 NBS Measurement Services:

Photometric Calibrations

R.L. Booker and D.A. McSparron

SP 250-15 NBS Measurement Services:

Calibration of X-Ray and Ganima-Ray

Measuring Instruments

P.J. Lamperti, T.P. Loftus, and R. Loevinger

SP 250-17 NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

SP 250-18 NBS Measurement Services: Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

SP 250-19 NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

SP 250-20 NBS Measurement Services:

Spectral Irradiance Calibrations

J.H. Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

SP 250-21 NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation and Sources

J.S. Pruitt, C.G. Soares, and M. Ehrlich
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300